This kind of temperature control is necessary in particular in the case of nitrogen oxide traps (NO.sub.x traps) such as are employed for exhaust gas purification in engines operating with a weak mixture (lean bum engines). While conventional three-way catalytic converters produce satisfactory conversion results over a relatively wide range of temperatures, and in the last few years it has been possible to reduce their sensitivity to overheating, known nitrogen oxide traps only operate in a satisfactory manner in a relatively restricted range of temperatures. At lower temperatures the nitrogen oxide trap does not absorb the pollutants efficiently enough. At too high temperatures the absorbed pollutants are partly set free again, thus impairing the purification efficiency. At still higher temperatures the nitrogen oxide trap can suffer accelerated ageing or even be permanently damaged. In view of the fact that under different engine conditions exhaust gas temperatures can vary in a range from 200 to 1000.degree. C., control of the exhaust gases flowing through the exhaust gas treatment system is necessary.
In German patent application 197 03 295.8, which was not published at the date of the present application, a method for controlling the temperature of a catalytic converter having a nitrogen trap is proposed in which the temperature of the nitrogen oxide trap is measured and the nitrogen oxide trap is operated in the weak mixture mode in a temperature range corresponding to the optimum efficiency of the nitrogen oxide trap by a device for varying the temperature of the exhaust gas. The device for varying the exhaust gas temperature can be an exhaust gas line having two pathways for the gas which have different heat losses from the exhaust gas and can be selected by means of a valve controlled by an actuating element. To perform the control it is proposed to employ a PI control element.
It has been found that the temperature sensor in the exhaust gas path should be located not before but preferably after at least one element of the nitrogen oxide trap. This arrangement has the particular advantage that if something goes wrong a threat of overheating can be investigated and prevented. Thus if ambient air penetrates into the exhaust system through a leak in the exhaust gas path before the nitrogen oxide trap, then in the case of engine operation with an enriched mixture (e.g. at full load) exothermal reactions occur with unburned components of the fuel, which can lead to overheating of the substrate in the nitrogen oxide trap. If the temperature sensor is located before the nitrogen oxide trap, the risk of such overheating cannot be detected.
However, locating the sensor after at least one element of the nitrogen oxide trap (e.g. between two elements [so-called "bricks"]), has the disadvantage that owing to the thermal inertia of these elements the control exhibits very long time lags. Thus a change in the exhaust gas temperature only leads to a corresponding change in the actual temperature measured at the temperature sensor after a delay of some 10 seconds. Such systems can only be controlled with difficulty by conventional PI controls, since unacceptable control oscillations occur. To handle such systems with long time lags so-called time lag controls have been proposed. However, in the case of time lag controls it is necessary to have a mathematical model of the system to be controlled and an advance estimate of the lag. Under unusual operating conditions, or if a fault develops, these models can fail, so that uncontrollable control response can occur.